The present invention relates to an antilock control device which can prevent the braking pressure from decreasing if a wheel speed detecting unit should malfunction.
A wheel speed detecting unit comprises, for example magnetic pickups or wheel speed sensors provided for the respective wheels of a motor vehicle to pick up the wheel speeds as AC signals, an interface circuit for converting the AC signals into pulse trains, and a timer/counter circuit and a processing circuit for converting the pulse trains into digital values or an F/V converter for converting the pulse trains into voltages representative of the wheel speeds.
There are various known methods for detecting the malfunction of such wheel speed detecting units. In one of such methods, a wheel speed detecting unit is judged to be malfunctioning if no output signals are detected by the time that the vehicle speed reaches a predetermined point. In another method, a detecting unit is judged to be malfunctioning if the difference among the signals from the wheel speed sensors is larger than a predetermined value after the vehicle speed has reached a predetermined point.
But if the pulse train representative of each wheel speed is normal at a given portion while at another portion, pulses are partially or completely lost, such a prior art malfunction detecting methods might not be able to detect any malfunction at all or might not be able to detect such a malfunction if the timing is bad.
The present inventors proposed an antilock control device (Japanese Patent Application 62-205830) provided with a warning means which is actuated if it is difficult to determine, only from the information supplied from the wheel speed detecting unit, whether there is something wrong with the detecting unit or the wheels are alternately falling into a locked state and recovering from locked state as a result of antilock control and if such a situation continues for a long time. But this device has a problem in that a long time is required for the judgement of a malfunction.
It is generally required that the vehicle speed at which the antilock control becomes inoperative be as low as possible, whereas another requirement is that a malfunction detection of the wheel speed detecting units be carried out only while the vehicle speed is within a relatively high range in order to prevent wrong judgements.
If the antilock control starts while the vehicle speed is within a low range, a request for antilock control will be given before the judgement on a malfunction of the wheel speed detecting unit is given. Thus, there is a possibility that the control may be actually started based on such a request in spite of the fact that it is a wrong request, or a permission to start the antilock control cannot be given until the vehicle speed gets rather high.